The present invention relates to the recording and reproducing of digitized composite SECAM signals and, more particularly, to the selected distribution of digitized composite SECAM samples into two digital recording or transmission channels, and to the recovery of the distributed pattern of recorded or transmitted samples, particularly in the event of a channel failure.
As is well known in the television industry, the SECAM color television standard used in France and Russia differs from the other world color television standards NTSC, PAL, and PAL-M. Briefly, instead of transmitting two chrominance signals simultaneously as in the other standards, the SECAM system transmits the chrominance signals sequentially. However, the two chrominance signals are required by the receiver simultaneously in order to produce the correct color. This is achieved in the SECAM system by storing the information of one line such that the same information then can be used for two lines which are adjacent in time. Since the chrominance signals are consecutively stored for a period of a line, and are used again for a respective adjacent line, during the period of any line both the chrominance signals are available simultaneously as required. Hence the proper chrominance signals can be combined together with the luminance signal, to produce the correct color signal.
Another distinction between the SECAM and other standards is that in the SECAM system the chrominance is encoded using FM modulation instead of quadrature, or phase, modulation as is used in NTSC, PAL and PAL-M. Since the frequency modulation process inherently is non-synchronous, and herein varies according to the modulating chrominance signal, the sampling rate of the SECAM system is non-synchronous to the chrominance information regardless of what sampling rate is selected.
Because of the special synchronous phase relationship of samples in NTSC,
and PAL-M systems, concealment algorithms with high performance are possible even in the event that large losses of data occur. For example, in the relatively new field of composite digital videotape recording systems (generally known as the D-2 format), the recording format employs two data recording channels and a respective pair of recording heads. Thus, a loss of a data channel such as may be caused, for example, by a clogged head, results in a 50% loss of data. The NTSC, PAL and PAL-M systems are able to conceal such large data losses. In SECAM however, as discussed above, there is no special phase relationship among the samples. It follows that, in the SECAM system, if a single sample is lost, or a number of samples significantly fewer than 50% of the data such as for example 10% are lost, a reasonably good filter can be designed to replace it based on the adjacent information on the same line of data. However, with a more extensive loss of data such as the loss of one channel in the two channel digital system of previous mention, or in the event of large dropouts of the order of from 10% to less than 50% of the data, full color concealment becomes impossible in the SECAM system.
More particularly, current composite digital videotape recorders such as those employing the D-2 format, distribute alternate samples to the two recording channels. When one channel of information is lost as when a head clogs, the SECAM color difference signal cannot be recovered because it is above the Nyquist limit of a single channel. Thus in such a two channel system, if every alternate sample is lost, a system using the SECAM standard cannot recover the chrominance information. This is due to the fact that in the SECAM system, the luminance information has a frequency spectrum of from zero to 3 megaHertz (MHz), while the chrominance information has a frequency spectrum of from 3.9 to 4.8 MHz. Given the usual sampling rate of approximately 16 MHz, then the Nyquist limit is 8 MHz. If alternate samples are arbitarily removed from the data stream, as in the example above where one channel of information is lost, the Nyquist limit is reduced to 4 MHz. Since the frequency band of from 4 to 5 MHz corresponding to the chrominance information is now lost, the chrominance information cannot be recovered in the SECAM system.
Accordingly, it would be highly desirable to provide a technique whereby a SECAM system readily can perform concealment of missing data, even in massive data loss approaching or equaling a 50% data loss such as when one of two channels of data is lost.
To this end, the invention contemplates, inter alia, the grouping of digital samples in select multiples per group, wherein alternate groups of samples are distributed for recording (or transmission) on alternate tracks via respective channels. By way of example only, it has been found that, given a sampling rate of the order of 16 MHz, the most effective and thus preferred number of samples per group is four. Thus, for reasons of simplifying the description, the invention combination herein is illustrated and described using a sample distribution pattern, and a luminance/chrominance data recovery algorithm, of four samples per group. However, given a sampling rate of the order of 12 MHz, the preferred number of samples per group is three, as further discussed below.
More particularly, given a SECAM system with a sampling frequency on the order of 16 MHz, grouping four adjacent samples together and distributing successive groups of four between alternate channels, causes a re-distribution of which frequency information is lost in the event of loss of a channel of data. That is, the use of groups of four samples on alternate channels, allows the recovery of luminance information in the spectrum of from zero to 1 MHz and chrominance information in the frequency spectrum of from 3 to 5 MHz. Thus in the SECAM system, the distribution of four samples per group in alternate recording tracks is, in effect, a compromise between the frequencies of the spectrum which are lost and those which are recoverable. For example, there is a loss in luminance information in the range of from 1 to 3 MHz, which constitutes mostly fine detail luminance information. Although the loss of detailed luminance causes a loss in resolution, the recovery of the low frequency luminance allows the recovery of a full color picture. On the other hand, the four sample distribution pattern provides a recoverable frequency spectrum of from 3 to 5 MHz, which advantageously allows recovering all the chrominance information between 3.9 and 4.8 MHz. Accordingly, even with a loss of half of the data due to the loss of a channel, sufficient luminance and chrominance information can be recovered in a SECAM system to allow acceptable concealment of the missing data.
The invention further contemplates receiving (for example, upon playback), the luminance and chrominance information via the sample distribution pattern, and performing the concealment process utilizing the multiple sample grouping and a selected algorithm, or algorithms. To this end, given the loss of one channel of data, in one algorithm the luminance and chrominance information is recovered by taking an average of the fourth previous sample and the fourth following sample on the line. In another algorithm which recovers further luminance information, in the simplest approach using only information on the same line as that having missing samples, the average is taken of all four previous and four following samples before and after the missing samples. For still higher resolution, vertical luminance detail may be obtained by first averaging groups of four adjacent surrounding samples before and after the missing data, and then averaging the four vertically adjacent surrounding samples above and below the missing samples.
The concealment technique then takes a weighted average of the resulting horizontal and vertical luminance averages to thereby recover luminance for the missing samples. The luminance and chrominance information then are combined to provide the SECAM full color signal.